Excess pressure in a raffinate splitter that accumulated during the restarting of a process unit has been identified by federal investiga-tors as the cause of a March 23 explosion that devastated the BP America refinery in Texas City, TX, leaving 15 dead and nearly 80 injured.

The following statement was issued April 7 by Bill Hoyle, investigation manager for the U.S. Chemical Safety and Hazard Investigation Board:

"The accident occurred during the restarting of process equipment known as the raffinate splitter, which separates hydrocarbons including pentane and hexane," the statement reads. "We believe that the raffinate splitter developed excess internal pressure, causing one or more pressure relief devices to open. When these devices opened, hydrocarbon liquid and vapor flowed into a vessel known as a blowdown drum. The blowdown drum includes a 100-foot-tall vent stack which goes to the atmosphere.

"The blowdown drum was not able to contain the hydrocarbon release, and it was not connected to a flare system to combust the flammable vapor. Instead, there was what witnesses describe as a geyser-like release of hydrocarbon liquid and vapor directly from the stack. The witnesses saw the liquid and vapor fall to the ground. Shortly afterward, the hydrocarbon ignited, and within seconds there was a powerful explosion, which resulted in deaths and injuries among personnel in nearby trailers and elsewhere.

"The exact ignition source for the hydrocarbon remains unknown, as does the cause of the excess pressure in the raffinate splitter. When our team entered the explosion site we found remnants of about 30 vehicles in close proximity to the vent stack, including some within 25 yards. However, further examination will be performed, and there are multiple ignition sources."

The site remains hazardous due to overhanging metal debris, the statement reads. There are also hazardous chemicals that remain trapped inside damaged process equipment.

"The process equipment will be depressurized, hanging debris will be removed, and a satellite control room will be made safe for entry," the statement reads.

The CSB team is also working on identifying blast markers in the affected area. Measuring the blast damage will assist investigators in reconstructing the size, nature, and origin of the explosion. The investigation is also delineating how far flammable vapors traveled through the area before the explosion.

Investigators are also working with Honeywell, the manufacturer of the computer control system in use, to recover computer records from the isomerization unit control room.

A subsequent statement issued by CSB board member John Bresland on April 28 states the following:

"The Board's investigation is continuing to focus on all the principal safety issues raised by the case including the design of the unflared blowdown system, the causes of the overpressure in the raffinate splitter, and the proximity of the trailers to an operating process unit. As most of you know, those trailers were 100-150 feet away from the blowdown drum when a geyser-like release of flammable hydrocarbons occurred on March 23.

"The trailers suffered extreme and catastrophic damage. The debris pattern at the site is complex and is not indicative of a single vapor cloud explosion. We believe that there were a number of distinct explosions in rapid succession, possibly as many as five. We are reviewing a variety of factors -- including the prevailing wind and topographic conditions -- to understand just how flammable vapors traveled from the blowdown drum to the area of the trailers.

"In particular, we are looking at the possibility of an explosion underneath the J.E. Merit double-wide trailer where a number of personnel were meeting. We are also examining whether this particular trailer had skirting around the base. The lack of skirting would have allowed flammable vapors to travel under the trailer.

"We hope to complete our field blast evaluation in the next one to two weeks. Thereafter there will be a period of computer-aided analysis. Ultimately, the information from this analysis will feed into our overall root-cause investigation.

"We are continuing to focus on the raffinate splitter column which was the original source of the flammable liquid and vapor that was released to the atmosphere. Specifically, we have been reviewing computer records from the control system equipment. Preliminary indications are that there was diminished flow of liquid out of the column. This column normally operates with 20-30 vertical feet of liquid at the bottom. In fact, we believe that the column likely became flooded, a highly abnormal condition.

"Over a period of several minutes, the pressure inside the column rose from about 20 pounds per square inch to about 60 pounds per square inch, which was also highly abnormal. This overpressure was more than sufficient to open three pressure relief valves on the column, which were set to open at 40 to 42 pounds per square inch. These valves remained open for six minutes, according to computerized records. During this period, it is assumed that sufficient liquid and vapor were discharged to overwhelm the blowdown system and cause the geyser-like release and the subsequent explosion. We will be adding several additional experts to our investigative team. These include experts in the design and operation of distillation columns and pressure-relief systems.

"Our investigation continues as we seek to further understand what precisely caused the flooding condition and the overpressure.

"In the course of our investigation, we also determined that there was a six-inch diameter drain line leading from the isom unit blowdown drum to a plant sewer. The manual valve on this line was found chained and locked open, and thus it is likely that a significant volume of flammable hydrocarbon traveled into the sewer from the blowdown drum. There is evidence that some flammable material was later released from the sewer at another location resulting in a fire, but at this time we cannot connect that fire with any of the fatalities.

"Finally, we have received reports of past instances where flammable vapor clouds were vented from unflared blowdown drums at the refinery. We are following up closely on these reports and hope to have further information at a future briefing."

On April 8, BP America released a list of steps it had taken to reinforce safety and prevent similar incidents from happening again. The actions include the following:

Completed a comprehensive review of every process unit's safety protection system. Any deviations found were addressed immediately or work has been discontinued until such deviations are addressed.

Relocated all personnel from temporary trailers, within 500 feet of a blowdown stack or flare. BP is in the process of removing all temporary trailers within this area.

Began relocating people whose jobs do not require them to be located near refinery equipment to reduce the potential consequences of refinery incidents.

Improved internal emergency communication planning with a focus on people first and communication about refinery operations in transition.

Started a review of all safety emergency systems including blow down drums and flares.